1. Field of the Invention
The present invention relates to a plant sensor capable of determining a growing condition of a crop or the like by irradiating measuring light toward the crop or the like of an object to be measured, and measuring the light intensity of the reflected light from the crop or the like. In particular, it relates to an improvement in a plant sensor capable of determining a growing condition of a crop or the like by obtaining a reflection rate according to the light intensity of the reflected light.
2. Description of the Related Art
In recent years, farmland areas have been decreasing due to changes in the environment, while the world's population has been increasing. Accordingly, farmland area per unit of the population has been decreasing, and it has been noted that the decrease in the farmland area per unit of the population will likely result in a food crisis.
Consequently, it has been requested to improve the production capacity of a crop per unit area, such as grains and vegetables. For this reason, it is important to accurately determine the growing condition of a crop, so as to effectively produce the crop.
Heretofore, an agricultural sensor such as a plant sensor has been proposed for determining a growing condition of a crop.
FIGS. 1A, 1B illustrate a general structure of a light source of a conventional agricultural sensor. In this agricultural sensor, the measuring light emitted from LEDs 44a-44e is irradiated to a crop, which is an object to be measured regarding a growing condition (hereinafter, referred to as an object to be measured), through a lens 30, and the light reflected from the crop is received by a first light receiver 38 via a concave mirror 34.
Moreover, in this agricultural sensor, part of the measuring light emitted from each of the LEDs 44a-44e is guided to a second light receiver 40 by a light-conductive plate 42, so as to be received by the second light receiver 40. This agricultural sensor measures the reflection rate of the crop according to the light-receiving signals of the first light receiver 38 and the light-receiving signals of the second light receiver 40.
This agricultural sensor also measures the reflection rate of the crop by using LEDs 46a-46e which emit measuring light whose wavelength is different from the wavelength of the measuring light emitted from the LEDs 44a-44e. 
The conventional agricultural sensor obtains a normalization difference vegetation index (NDVI) according to the reflection rates of the two wavelengths which are different from each other (for example, refer to U.S. Pat. No. 6,596,996B1, registration date Jul. 22, 2003).
According to the conventional agricultural sensor, fertilizer can be effectively spread by obtaining the information regarding the growing condition of the crop.
By the way, in the conventional agricultural sensor, the light intensity received by the first light receiver 38 contains a component resulting from ambient light and a component according to the measuring light reflected from a crop.
In this conventional agricultural sensor, the component resulting from the ambient light is eliminated by means of a band-pass filter 32.
However, in this conventional agricultural sensor, the structure of the optical system is slightly complicated, and also the component resulting from the ambient light can not be completely eliminated even if the band-pass filter is used. Therefore, it becomes difficult to accurately measure the reflection rate of the crop.
In this conventional agricultural sensor, since the wavelength and the light-emitting intensity of the light source are changed depending on an environmental temperature, it is difficult to accurately measure the reflection rate of the crop.